staging: comedi: drivers: have core hook up default (*insn_read) for readback

Most of the comedi drivers that provide readback for write only subdevices now
use the comedi core comedi_alloc_subdev_readback() helper to allocate the subdevice
'reaback' member instead of using some member in their private data. These drivers
also hook up the (*insn_read) callback to the comedi_readback_insn_read() helper to
provide the readback.

Have the core automatically hook up the (*insn_read) callback after allocating the
memory if the driver has not already hooked it up to a private function.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>